bug#62940: 29.0.60; vc: no easy way to get diff of all outgoing changes

[Eli Zaretskii]

> From: sbaugh@catern.com
> Date: Sat, 22 Apr 2023 13:00:48 +0000 (UTC)
> Cc: sbaugh@janestreet.com, 62940@debbugs.gnu.org, fgunbin@fastmail.fm
> 
> 1. View a file buffer
> 2. Hit C-x v O
> 3. See outgoing changes only for that file

I don't think it is TRT to change the behavior of "C-x v O" in the
above scenario.  Because it will make it much harder to see the
changes for all the files, since in Emacs you are almost always "in
some file-visiting buffer".

So the above can only be optional behavior, not the default.